Environmental Monitoring

How do i calibrate a remote microphone system?

Condenser microphones are extremely stable and sensitive transducers when used in their normal operating conditions. When used for long-term noise monitoring, they can be exposed to more extreme environments (e.g. high temperatures, high windspeeds, moisture, etc) which may cause a change in sensitivity, and even damage. The nature of this type of monitoring often precludes a site visit to calibrate the microphone in the normal way, such as using a sound level calibrator or pistonphone mounted directly on the microphone capsule.

We therefore need a method of checking that all is well at the microphone end, so this article describes two methods that can be used.

SysCheck

The first is SysCheck from GRAS, which simply injects an electrical signal into the microphone circuit, via the preamplifier, to check the signal path integrity. This includes the microphone capsule itself, which means that any change in the resulting measured signal can be used to deduce if the microphone capacitance has changed, which might be an indicator of damage (e.g. damage or corrosion on the diaphragm, or physical damage). A similar method is also called charge injection calibration (CIC).

It’s important to note that this is not a ‘calibration’ as such, it is simply a means to check that nothing has changed out of tolerances, which can be preset by the measuring system. It is not an acoustical signal.

In order to inject the signal, an additional connection is required on the preamplifier, and this is normally available via an industry standard 7-pin Lemo connector, used by many GRAS power supplies, such as 12AK, 12AA etc. These power supplies include the signal generator, and will activate the signal when the Syscheck button is pressed, or remotely by software trigger. This method therefore precludes a simple co-axial connection to the microphone, such as IEPE, which is used solely to provide power to the preamplifier, and return the measured signal. Some front-ends have Syscheck built-in, such as Apollo from Sinus, where the Samurai software can provide the necessary signal on the correct pin of the Lemo connector. For this method, a suitable preamplifier must be used, such as the 26AJ from GRAS, and this feature is also built-in to the GRAS 41AC-2 Environmental Microphone.

Electrostatic Actuation

A more stable and repeatable calibration can be achieved by using an electrostatic actuator mounted on the microphone itself. This takes the form of a plate mounted very close to the microphone diaphragm, and normally replaces the standard protection grid. Some microphones incorporate the actuator in the weather protection system (e.g. rain cover). A variant is to electrically isolate the top plate of the standard microphone grille, so this doubles as the actuator (e.g. the MK255 capsule in the Svantek SV200).

A signal (typically at 1kHz) is supplied by a generator, via a special amplifier, to create an electrostatic modulation of the microphone diaphragm. This is effectively an acoustic signal, so it checks not only the integrity of the microphone, but also the sensitivity. This is similar to the way microphones are calibrated in the laboratory.

This method therefore requires the generator and amplifier to drive the actuator, and is completely separate to the signal and powering chain of the microphone. Outdoor microphones such as the GRAS 41CN have all the necessary electronics integrated, and the actuation can be triggered by a simple contact closure on a serial port for example. This method is used in many long-term monitoring systems, such as the Sinus Swing.

The SV200 from Svantek is a complete outdoor noise analyser, and has the necessary system built-in. The electrostatic calibration can be triggered via a web page, either manually or automatically at predetermined intervals.

Although this method can be used as a ‘calibration’, the sensitivity of the system is not normally adjusted, but the levels logged to ensure accuracy of the results.Again, because the method requires external electronics and connections, it is not possible to do this via a single co-axial connection such as IEPE.

Ground vibration monitoring

How big a USB memory stick can I use with the USB host facility of 95x series instruments?

In general, the larger the USB memory, the larger the current requirements, and the slower the speed of the memory. The Svan 95x are low power devices, so have an (intentionally) limited supply of power to the USB host. Therefore, we recommend that the maximum memory size is 4GB, as this provides the best compromise between power, speed and capacity. Larger memory sizes may work perfectly well, but we can’t guarantee it.

Also, we recommend good brands such as Kingston, rather than supermarket own-brand.

So why should I choose the Svan958A over the SV106?

If you want to measure noise, have remote communications and measure dominant frequency, then Svan 958A is the best choice

Does the SV106 do FFT?

No, but 1/1 and 1/3 octave options are available

Does the SV106 do PPV and VDV at the same time?

Yes, each channel has two profiles, so you can measure unweighted PPV and weighted VDV at the same time

What about the SV106? Can I use that for ground vibration?

Yes, the 106 is a six channel instrument so two triaxial locations can be measured

Can the Svan958A measure Vibration Criteria?

Yes, with the normal SV84(SV207B) triaxial accelerometer, and 1/3 octaves installed, you can measure VC-A to VC-D, but for measurements to VC-E and NIST-A, we recommend using a 10V/g accelerometer instead, as that gives you more headroom over the noise floor. Typically, three accelerometers are mounted orthogonally in an SA217 block – ask for details.

Can the Svan958A measure according to the criteria in BS7385?

Yes, if the FFT option is installed, then PPV and dominant frequency can be stored and displayed correctly in SvanPC++ with the threshold criteria.

Can I trigger from a vibration event and send an email/SMS?

Yes, if the 958A is equipped with a serial interface and modem (options), then two trigger levels are available on each channel, typically used for warning and alarm levels.

Can I measure noise at the same time as triaxial vibration on the SV958A?

Yes – we use channels 1-3 for vibration, and channel 4 can be used for noise measurements to Class 1 accuracy. Measurements include Leq and Ln as standard, and spectra (if option installed)

Can I capture the event signal file on the Svan958A?

Yes, if the WAV option is installed, the ground vibration mode will also capture the signal file to a USB memory stick.

Can I measure dominant frequency to BS7385 on the Svan958A?

Yes, if FFT is installed, the ground vibration mode allows storage of dominant frequency. This is preferred over zero-crossing methods, as it reduces errors caused by complex time waveforms.

Can I measure VDV and PPV at the same time as 1/3 octaves?

In the special ground vibration mode, 1/3 octave analysis is not possible at the same time as PPV and VDV, but for frequency content, FFT (option) can be used.

Can I measure VDV and PPV at the same time?

Yes, in the special ground vibration mode (included as standard), you can measure both at the same time, although ask yourself why you need to do this – normally VDV is for vibration nuisance/disturbance and PPV is for building damage, so they tend to be measured in different places.

Can I measure 1/3 octaves at the same time as PPV?

Yes, but you need the 1/3 octave option installed.

Can I measure 1/3 octaves at the same time as VDV?

Yes, but you need the 1/3 octave option installed.

Can I measure a VDV time history on the Svan958A?

Yes, you can log individual VDV values with time, e.g. for train pass-bys etc. They can then be accumulated in the SvanPC++ software. Periodic (e.g. hourly) VDV values can be stored at the same time.

Can the Svan958A measure VDV?

Yes, it measures VDV according to BS6472, and you can select Wd weighting for the horizontal axes and Wb for the vertical axis. Wg is also available for backwards compatibility with the old version of the standard.

Can the Svan958A measure PPV?

Yes, it measures PPV directly, with selectable high pass filters of 1, 3 and 10Hz. We recommend 3Hz filter for measurements to BS7385

Human vibration monitoring

What's all that IEPE, CCP stuff mean?

The output of the crystal is a charge, which requires a specialised charge amplifier, with extremely high input impedance, in order to drive our measuring system. These used to be separate boxes, with specialised low-noise cabling, but nowadays, the charge amplifier is built into the accelerometer itself, and this uses a ‘phantom’ powering system known as IEPE (integrated electronic piezo-electric), also known by a variety of proprietary names such as ICP®, CCP etc. At least IEPE is standardised! This means that long cables can be driven, and as long as your instrument can provide the powering, you should be in business. But always check that you have an IEPE accelerometer rather than a charge accelerometer first!
Due to being a capacitor, such accelerometers do not have a DC response, and will roll-off at low frequencies. Make sure you select one suitable for your task, if you want to measure down to 0.5Hz for example.

How does a standard accelerometer work?

The majority of accelerometers for our applications are piezoelectric devices. A small piezoceramic crystal is sandwiched between the base and a seismic mass, so when the base is accelerated, the crystal is stressed, causing a proportional charge output. Because it is a simple mass/spring system, it will have a fundamental resonance – the crystal is very stiff, so this will be high, some kilohertz for most devices. Below that resonance, the response is virtually flat and linear, making an excellent transducer.
To make a sensitive accelerometer, make the mass and/or crystal bigger – but, this brings the resonance down, so there’s a trade-off to be made. Thankfully, most requirements for sensitive accelerometers are at low frequencies!

How do you measure vibration?

Vibration transducers can be split basically into two types – accelerometers and geophones (or seismometers). Accelerometers have an output proportional to acceleration, and geophones have an output proportional to velocity. So how can both be used to measure vibration?
There’s a basic relationship between acceleration and velocity – the former being the rate of change, or the differential, of velocity. Therefore we can easily convert between the two by integrating an acceleration signal to yield a velocity signal. This is normally done in the time domain, using a filter (called an integrator), but it can also be done in the frequency domain by dividing an acceleration spectrum by 2πf, where f is the frequency. This effectively slopes the spectrum by -6dB/octave, so a velocity spectrum will appear to have a lot fewer high frequency components

What's contact force and why do i need to measure it?

Contact force is a measurement of the amount of force between a user’s hand and the tool they’re using. The SV103 personal vibration dosimeter and the SV06 (when used with the SV105AF transducer) are unique in that they measure contact force, which ensures any uncertainty of the measurement to be greatly reduced. You can choose to disregard any data where there is insufficient contact force.

The new dosemeter standard, under preparation and until recently chaired by Paul Pitts, will specify a new class of instrument called a vibration dosemeter, which will require contact force measurement. Svantek have the only instruments which currently meets the draft standard, and the SV103 and SV106 are the only ones on the market for now.

Does the hand mounted vibration accelerometer comply with regulations?

There are two standards, ISO 8041:2005 and ISO 5349:2001 Parts 1 & 2

ISO 8041 covers the instrument specification, so things like frequency response, weighting filters, detector linearity, measurement parameters etc.

The SV103 meets the standard.

ISO 5349 is a procedural standard, which tells you what you need (i.e. an instrument which meets ISO 8041), what to measure (e.g. AEQ, triaxial orientation of the hand, etc) and how to do the measurement (practical considerations, what to report, etc).

The SV103 meets both parts of the standard.

The most common misinterpretation of ISO 5349 is that I have heard people saying you cannot use hand-held accelerometers for the measurement. It does not say this in the standard. It discusses both tool-mounted and hand mounted, and in fact, with many tools (e.g. sanders, polishers, planers etc) it is not possible to use a tool-mounted accelerometer.

Personal noise dosimetry

Why would I want octave band filters on a noise dosimeter?

The answer is simple really – if you want to correctly select hearing protection for your workers.

There are a number of ways to take measurements to help select hearing protection including C-A and HML measurements but these are all compromises compared to taking correct octave band measurements. The SV104 is the only personal noise dosimeter on the market that has octave band filters as an option.

SV106

So why should I choose the Svan958A over the SV106?

If you want to measure noise, have remote communications and measure dominant frequency, then Svan 958A is the best choice

Can I measure WAV files with SV106?

Yes, recorded to internal microSD card, for post-processing in SvanPC++

Does the SV106 do noise?

No. It’s a vibration meter only.

Does the SV106 do FFT?

No, but 1/1 and 1/3 octave options are available

Does the SV106 do PPV and VDV at the same time?

Yes, each channel has two profiles, so you can measure unweighted PPV and weighted VDV at the same time

What about the SV106? Can I use that for ground vibration?

Yes, the 106 is a six channel instrument so two triaxial locations can be measured

SVAN958A

How big a USB memory stick can I use with the USB host facility of 95x series instruments?

In general, the larger the USB memory, the larger the current requirements, and the slower the speed of the memory. The Svan 95x are low power devices, so have an (intentionally) limited supply of power to the USB host. Therefore, we recommend that the maximum memory size is 4GB, as this provides the best compromise between power, speed and capacity. Larger memory sizes may work perfectly well, but we can’t guarantee it.

Also, we recommend good brands such as Kingston, rather than supermarket own-brand.

So why should I choose the Svan958A over the SV106?

If you want to measure noise, have remote communications and measure dominant frequency, then Svan 958A is the best choice

Can the Svan958A measure Vibration Criteria?

Yes, with the normal SV84(SV207B) triaxial accelerometer, and 1/3 octaves installed, you can measure VC-A to VC-D, but for measurements to VC-E and NIST-A, we recommend using a 10V/g accelerometer instead, as that gives you more headroom over the noise floor. Typically, three accelerometers are mounted orthogonally in an SA217 block – ask for details.

Can the Svan958A measure according to the criteria in BS7385?

Yes, if the FFT option is installed, then PPV and dominant frequency can be stored and displayed correctly in SvanPC++ with the threshold criteria.

Can I trigger from a vibration event and send an email/SMS?

Yes, if the 958A is equipped with a serial interface and modem (options), then two trigger levels are available on each channel, typically used for warning and alarm levels.

Can I measure noise at the same time as triaxial vibration on the SV958A?

Yes – we use channels 1-3 for vibration, and channel 4 can be used for noise measurements to Class 1 accuracy. Measurements include Leq and Ln as standard, and spectra (if option installed)

Can I capture the event signal file on the Svan958A?

Yes, if the WAV option is installed, the ground vibration mode will also capture the signal file to a USB memory stick.

Can I measure dominant frequency to BS7385 on the Svan958A?

Yes, if FFT is installed, the ground vibration mode allows storage of dominant frequency. This is preferred over zero-crossing methods, as it reduces errors caused by complex time waveforms.

Can I measure VDV and PPV at the same time as 1/3 octaves?

In the special ground vibration mode, 1/3 octave analysis is not possible at the same time as PPV and VDV, but for frequency content, FFT (option) can be used.

Can I measure 1/3 octaves at the same time as PPV?

Yes, but you need the 1/3 octave option installed.

Can I measure 1/3 octaves at the same time as VDV?

Yes, but you need the 1/3 octave option installed.

Can I measure a VDV time history on the Svan958A?

Yes, you can log individual VDV values with time, e.g. for train pass-bys etc. They can then be accumulated in the SvanPC++ software. Periodic (e.g. hourly) VDV values can be stored at the same time.

Can the Svan958A measure VDV?

Yes, it measures VDV according to BS6472, and you can select Wd weighting for the horizontal axes and Wb for the vertical axis. Wg is also available for backwards compatibility with the old version of the standard.

Can the Svan958A measure PPV?

Yes, it measures PPV directly, with selectable high pass filters of 1, 3 and 10Hz. We recommend 3Hz filter for measurements to BS7385